1. Field of the Invention
This invention relates to a charging system for charging a member to be charged, a process cartridge and an image-forming apparatus, and these are preferably used in copying machines and printers of the electrophotographic type and the electrostatic-recording type. As the member to be charged, use can be made of a photosensitive member or a dielectric member.
2. Description of Related Art
(A) Process Cartridge
Heretofore, in an electrophotographic image-forming apparatus using an electrophotographic image forming process, there has been adopted a process-cartridge system in which an electrophotographic photosensitive member as an image-bearing member, which is a member to be charged, and at least one of charging means, developing means and cleaning means as process means for acting on the electrophotographic photosensitive member are integrally made into a process cartridge, which is made detachably mountable with respect to an electrophotographic image-forming apparatus main body.
According to this process-cartridge system, the maintenance of the image-forming apparatus can be done by a user himself without resort to a serviceman and therefore, operability can be markedly improved. So, this process-cartridge system is widely used in electrophotographic image-forming apparatuses. In such a process-cartridge system, operability for a user and a lower cost are desired and therefore, it is desirable to make particularly the diameter of a photosensitive member, which is an image-bearing member, small.
(B) Toner-Recycle Process (Cleanerless System)
In an image-forming apparatus of a transfer type, any untransferred residual developer (untransferred toner) on an image-bearing member after transfer is removed from the surface of the image-bearing member by a cleaning apparatus (cleaner) and becomes waste toner, and it is desirable from the viewpoint of environmental protection that this waste toner be not discarded.
So, there has appeared even an image-forming apparatus using a toner-recycle process (toner-recycle system or cleanerless system) which is of an apparatus construction in which an exclusive cleaner is eliminated, and any untransferred toner on an image-bearing member after transfer is removed from the image-bearing member by “cleaning simultaneous with developing” by a developing apparatus, and is collected for reuse in the developing apparatus.
“Cleaning simultaneous with developing” is a method of collecting any residual toner on an image-bearing member after transfer during developing subsequent to the next step, i.e., in an electrophotographic process, when continuing to charge the image-bearing member, exposing it to thereby form a latent image, and developing this latent image, by a fog-removing bias (a fog-removing potential difference V back which is the potential difference between a DC voltage applied to a developing apparatus and the surface potential of the image-bearing member).
According to this method, the untransferred toner is collected into the developing apparatus and reused at the next and subsequent steps and therefore, waste toner can be eliminated and the cumbersomeness for maintenance can be reduced. Also, because it is cleanerless, the advantage in space is also great and it becomes possible to greatly downsize the image-forming apparatus or the process cartridge.
In the toner-recycle process, the untransferred toner is once introduced into a contact-charging member and is made to enter a reusable state (the original charge amount of the toner) and is returned to the developing apparatus through the intermediary of the image-bearing member, whereby it is again used for developing, or if unnecessary, is collected. As a result, toner recycling becomes possible. In a charging apparatus used here, besides the charging of the image-bearing member, the collection of the untransferred toner and the recharging of the toner become necessary.
(C) Particle Charging (Powder-Applying Type)
There have been proposed a charging apparatus (a powder-applying-type, direct injection-charging apparatus) for charging an image-bearing member by direct injection charging with non-magnetic electroconductive particles being present in a charging-contact part (nip part) formed by the image-bearing member and a contact-charging member, and an image-forming apparatus using a toner-recycling process (a cleanerless system) utilizing the same. In this system, in order to supply electroconductive particles to the charging-nip part, it is desirable to supply the electroconductive particles from a developing device to the image-bearing member, and consequently, it is preferable to be of a cleanerless type in order that the electroconductive particles supplied to the image-bearing member may be stably supplied to the charging-nip part.
A suitable amount of the electroconductive particles contained in a developer in the developing apparatus shifts to the image-bearing member together with the toner during the development of an electrostatic latent image. A toner image on the image-bearing member is attracted and positively shifts to a transfer-material side in a transferring-nip part under the influence of a transferring bias, but the electroconductive particles on the image-bearing member, due to their being electroconductive, do not positively shift to the transfer-material side, and substantially adhere to and are retained on the image-bearing member and are residual thereon. The residual electroconductive particles on the surface of the image-bearing member after the transfer of the toner image to the transfer material are intactly brought to the charging-nip part with the untransferred toner by the rotation of the image-bearing member.
In this manner, the charging of the image-bearing member is effected with the electroconductive particles being present in the charging-nip part.
Owing to this presence of the electroconductive particles, the elaborate contacting property of a charging roller as a contact-charging member with the image-bearing member and the contact resistance thereof can be maintained and therefore, the direct injection charging of the image-bearing member by the charging roller can be effected. That is, the charging roller closely contacts the image-bearing member with the electroconductive particles interposed therebetween and the electroconductive particles present in the charging-nip part rub closely against the surface of the image-bearing member, whereby high charging efficiency can be obtained by stable and safe direct injection charging which does not use a discharging phenomenon, and a potential substantially equal to a voltage applied to the charging roller can be given to the image-bearing member.
Also, the untransferred residual toner on the surface of the image-bearing member after the transfer of the toner image to the transfer material is not removed by a cleaner, but comes to a developing portion via the charging-nip part with the rotation of the image-bearing member, and is cleaned (collected) simultaneously with developing by the developing apparatus. The untransferred toner arriving at the charging-nip part by the rotation of the image-bearing member, and adhering to and mixed with the charging roller is gradually discharged from the charging roller onto the image-bearing member, and comes to the developing portion with the movement of the surface of the image-bearing member, and is cleaned (collected) simultaneously with developing by the developing apparatus. A charging process thus using the electroconductive particles is suited for a cleanerless process having no exclusive cleaner, and due to the absence of the cleaner, a reduction in the diameter of the image-bearing member can also be expected and therefore, this charging process is a technique also effective to achieve a lighter weight and lower cost process cartridge.